Spin-based electronics or spintronics exploit both the charge of electrons as well as the spin of the electrons to permit new devices with enhanced functions, higher speeds, and/or reduced power consumption, for example. An exemplary spintronic device is the spin valve as illustrated in the FIGS. 1A and 1B. The spin valve 11 provides a low resistance when the spins are aligned (FIG. 1A), and provides a high resistance with the spins not aligned (FIG. 1B). The spin valve 11 may be used as a nonvolatile memory element, for example. Other exemplary spintronic devices including the spin-FET 12 schematically illustrated in FIG. 2, and the quantum bit device 13 illustrated in FIG. 3.
Published U.S. Patent Application No. 2006/0018816, for example, discloses a Diluted Magnetic Semiconductor (DMS) comprising zinc oxide which includes a transition element or a rare earth lanthanide, or both, in an amount sufficient to change the material from non-magnetic state to a room temperature ferromagnetic state. The material may be in a bulk form or a thin film form. A DMS material is a semiconductor in which transition metal ions or rare earth lanthanides substitute cations of host semiconductor materials. More particularly, a DMS material 15 is schematically illustrated in FIG. 4B, while to the left in FIG. 4A is a magnetic material 14, and to the right in FIG. 4C is a non-magnetic material 16.
Published U.S. Patent Application No. 2005/0258416 discloses a spintronic switching device comprising a half-metal region between first and second conductive regions. The half-metal region comprises a material that, at the intrinsic Fermi level, has substantially zero available electronic states in a minority spin channel. Changing the voltage of the half-metal region with respect to the first conducting region moves its Fermi level with respect to the electron energy bands of the first conducting region, which changes the number of available electronic states in the majority spin channel. In doing so, this changes the majority spin polarized current passing through the switching device. The half-metal region may comprise CrAs and the conducting regions may comprise a p-doped or n-doped semiconductor. For example, the p-doped semiconductor may comprise Mn doped GaAs.
Published U.S. Patent Application No. 2004/0178460 discloses a spintronic device application as a memory and a logic device using a spin valve effect obtained by injecting a carrier spin-polarized from a ferromagnetic into a semiconductor at room temperature, and a spin-polarized field effect transistor. The ferromagnet is disclosed as one of a Fe, Co, Ni, FeCo, NiFe, GaMnAs, InMnAs, GeMn, and GaMnN, and can be a half metal having a spin polarization of 100% such as CrO2. The semiconductor may be one selected from Si, GaAs, InAs, and Ge. Also, the spin channel region is disclosed as Si on insulator (SOI) or a two-dimensional electron gas of a compound semiconductor.
An article to Jonker et al. “Electrical Spin Injection and Transport in Semiconductor Spintronic Devices”, MRS Bulletin/October 2003, pp. 740-748, discloses semiconductor heterostructures that use carrier spin as a new degree of freedom. The article discloses four essential requirements for implementing a semiconductor spintronics technology in devices, and provides that the efficient electrical injection of spin-polarized carriers into the semiconductor has been a critical issue severely hampering progress in this field. The article further discloses that advances in materials quality have increased the Curie temperature of Ga1−xMnxAs to ˜150° K with the potential of exceeding room temperature. Spin-dependent resonant tunneling is identified as able to increase the spin selectivity of tunneling contacts in a very efficient way. A double-barrier heterojunction (DBH) comprising a nonmagnetic semiconductor quantum well between two insulating barriers and two ferromagnetic semiconductive electrodes may behave as half-metallic junctions if the parameters of the quantum well and barrier are properly tuned.
Current spintronics technology is limited by the currently used materials. For example, it is important, as noted by Jonker et al., to have efficient spin carrier injection. It is also desirable to have manufacturing and operational compatibility with existing semiconductor processing technology. It is also desirable that the magnetic ordering or Curie temperature by at or near room temperature, instead of the more typical 100-200° K. One potential approach is the DMS materials as disclosed in the above noted U.S. Patent Application No. 2004/0178460.
Another spintronic device structure is the Digital Ferromagnetic Heterostructure (DFH) as disclosed, for example, by Sanvito et al. in an article “Ab Initio Transport Theory for Digital Ferromagnetic Heterostructures” in Physical Review Letters, Vol. 87, No. 26, Dec. 24, 2001, pp. 1-4. The article notes that the solubility limit of Mn in GaAs is rather small; however, a large MN concentration can be obtained in a zinc blende MnAs submonolayers into GaAs to form a MnAs/GaAs superlattice. A schematic diagram of a prior art DFH structure 18 is shown in FIG. 5 with a transition metal (Tm) in the form of Mn within a Silicon superlattice. Although this may have a large spin polarization at the Fermi level and a large magnetoresistance effect and Curie temperature higher than in the bulk, it may suffer from a low thermal stability.
Unfortunately, many of the materials and structures for spintronic devices have relatively low concentrations of the spintronic dopant, such as Mn. The spintronic dopant tends to precipitate out of the crystal lattice, especially as the concentration is increased, and/or the device is subjected to thermal processing steps.